The minimum metallicity of globular clusters and its physical origin -- implications for the galaxy mass-metallicity relation and observations of proto-globular clusters at high redshift

Abstract

In the local Universe, globular clusters (GCs) with metallicities $[\mathrm{Fe}/\mathrm{H}]<-2.5$ are extremely rare. In this Letter, the close connection between GC formation and galaxy evolution is used to show that this GC metallicity `floor' results from the galaxy mass-metallicity relation of ultra low-luminosity galaxies (ULLGs) at high redshift, where the most metal-poor GCs must have formed. Galaxies with metallicities $[\mathrm{Fe}/\mathrm{H}]\lesssim -2.5$ have too low masses to form GCs with initial masses $M_{\mathrm{i}}\gtrsim 10^5{\mathrm{M}}_{\odot }$, needed to survive for a Hubble time. This translates the galaxy mass-metallicity relation into a maximum initial cluster mass-metallicity relation for $[\mathrm{Fe}/\mathrm{H}]\lesssim -1.8$, which naturally leads to the observed colour-magnitude relation of metal-poor GCs at $z=0$ (the `blue tilt'). Its strength traces the slope of the gas phase mass-metallicity relation of ULLGs. Based on the observed blue tilt of GCs in the Virgo and Fornax Clusters, the galaxy mass-metallicity relation is predicted to have a slope of $\alpha =0.4\pm 0.1$ for $10^5\lesssim M_{\star }/{\mathrm{M}}_{\odot }\lesssim 10^7$ at $z\gtrsim 2$. The GC metallicity floor implies a minimum host galaxy mass and a maximum redshift for GC formation. Any proto-GCs that may be detected at $z>9$ are most likely to end up in galaxies presently more massive than the Milky Way, whereas GCs in low-mass galaxies such as the Fornax dSph ($M_{\star }\approx 4\times 10^7{\mathrm{M}}_{\odot }$) formed at $z\lesssim 3$.

5 pages, 6 figures; MNRAS Letters in press (accepted 2019 April 10; received 2019 April 9; in original form 2019 March 17)

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